Method and apparatus for non-destructive testing

ABSTRACT

Non-destructive testing apparatus may comprise a photon source and a source material that emits positrons in response to bombardment of the source material with photons. The source material is positionable adjacent the photon source and a specimen so that when the source material is positioned adjacent the photon source it is exposed to photons produced thereby. When the source material is positioned adjacent the specimen, the specimen is exposed to at least some of the positrons emitted by the source material. A detector system positioned adjacent the specimen detects annihilation gamma rays emitted by the specimen. Another embodiment comprises a neutron source and a source material that emits positrons in response to neutron bombardment.

CONTRACTUAL ORIGIN OF THE INVENTION

This invention was made with United States Government support underContract No. DE-AC07-99ID13727 awarded by the United States Departmentof Energy. The United States Government has certain rights in theinvention.

TECHNICAL FIELD

This invention relates generally to the testing and evaluation ofmaterials and more specifically to methods and apparatus for performingnon-destructive testing of materials using position annihilation.

BACKGROUND

Non-destructive material evaluation refers to any of a wide variety oftechniques that may be utilized to examine materials for defects and/orevaluate the materials without requiring that the materials first bedestroyed. Such non-destructive material evaluation is advantageous inthat all materials or products may be tested for defects. After beingevaluated, acceptable (e.g., substantially defect-free or withacceptable defect levels) materials may be placed in service, while thedefective materials may be re-worked or scrapped, as may be appropriate.Non-destructive evaluation techniques are also advantageous in thatmaterials already in service may be evaluated or examined in-situ,thereby allowing for the early identification of materials or componentsthat may be subject to in-service failure. The ability to evaluate orexamine new or in-service materials has made non-destructive materialevaluation techniques of great importance in safety- orfailure-sensitive technologies, such as, for example, in aviation andspace technologies, as well as in nuclear systems and in powergeneration systems.

One type of non-destructive evaluation technique, generally referred toas positron annihilation, is particularly promising in that it istheoretically capable of detecting fatigue and other types of damage inmetals, composites, and polymers at its earliest stages. While severaldifferent positron annihilation techniques exist, as will be describedbelow, all involve the detection of positron annihilation events inorder to ascertain certain information about the material or objectbeing tested.

By way of background, complete annihilation of a positron and anelectron occurs when both particles collide and their combined mass isconverted into energy in the form of two (and occasionally three)photons (e.g., gamma rays). If the positron and the electron are both atrest at the time of annihilation, the two gamma rays are emitted inexactly opposite directions (e.g., 180° apart) in order to satisfy therequirement that momentum be conserved. Each annihilation gamma ray hasan energy of about 511 keV, the rest energies of an electron and apositron.

In positron annihilation analysis, the momentum of the electron isrelated to the environment in which it resides. For example, electronmomentum is relatively low in defects in metals or in microcracks incomposite materials and polymers) or in large lattice structures.Electron momentum is higher in defect-free or tight lattice structures.One way to determine the momentum of the electron is to measure thedegree of broadening of the gamma-ray energy peak in the spectrum aroundthe 511 keV annihilation energy produced by the annihilation event.Alternatively, the momentum of the electron may be derived from thedeviation from 180° of the two annihilation gamma rays.

Additional information about the electron density of the material at thesite of annihilation may be obtained by determining the average lifetimeof the positrons following a known initiation event before they areannihilated. Still other information about the annihilation event may bedetected and used to derive additional or supplemental informationregarding the material being tested, such as the presence ofcontaminants or pores within the material. Accordingly, the detection ofpositrons and the products of annihilation events provide muchinformation relating to defects and other microstructuralcharacteristics of the material or object being tested.

As mentioned above, several different positron annihilation techniquesare known. In one type of positron annihilation technique, positronsfrom a radioactive source (e.g., ²²Na, ⁶⁸Ge, or ⁵⁸Co) are directedtoward the material to be tested. Upon reaching the material, thepositrons are rapidly slowed or “thermalized.” That is, the positronsrapidly loose most of their kinetic energy by collisions with ions andfree electrons present at or near the surface of the material. Afterbeing thermalized, the positrons then annihilate with electrons in thematerial. During the diffusion process, the positrons are repelled bypositively-charged nuclei, thus tend to migrate toward defects such asdislocations in the lattice sites where the distances topositively-charged nuclei are greater. In principle, positrons may betrapped at any type of lattice defect having an attractive electronicpotential. Most such lattice defects are so-called “open-volume” defectsand include, without limitation, vacancies, vacancy clusters,vacancy-impurity complexes, dislocations, grain boundaries, voids, andinterfaces. In composite materials or polymers, such open-volume defectsmay be pores or microcracks.

Positron annihilation techniques utilizing external positron sourcessuffer from a variety of disadvantages. For example, one type ofexternal positron source is an isotopic source, such as ²²Na, whichemits positrons having generally low energies of about 0.5 millionelectron volts (MeV) or so. Such low energy positrons can only penetratea short distance, e.g., less than about 10 microns or so, into metallicmaterials. Such positrons also lose energy in the source materialitself. While higher energy positrons (e.g., having energies of about 3MeV) can be obtained via non-isotopic sources, such as the Pelletron atLawrence-Livermore Laboratories, such devices are also not without theirdisadvantages. For example, the positron beams produced by such sourcesare often relative narrow, thus cannot easily be made to cover largerspecimens. In addition, such external positron sources tend to bephysically large, which can limit the ability to place the positronsource at the appropriate location on the specimen to be tested. This isparticularly true if the specimen comprises a fabricated structure(e.g., a wing structure) that comprises small areas or regions that aresimply not large enough to accommodate the large external positronsource. Consequently, it may be difficult, if not impossible, toeffectively test such structures.

SUMMARY OF THE INVENTION

Non-destructive testing apparatus may comprise a photon source and asource material that emits positrons in response to bombardment of thesource material with photons. The source material is alternatelypositionable adjacent the photon source and a specimen. When the sourcematerial is positioned adjacent the photon source, the source materialis exposed to photons produced by the photon source, which generatespositron-producing nuclei within the source material. When the sourcematerial is positioned adjacent the specimen, the specimen is exposed toat least some of the positrons being emitted by the source material. Adetector system positioned adjacent the specimen detects annihilationgamma rays emitted by the specimen that provide a measure of themicrostructure of the specimen.

Another embodiment of the non-destructive testing apparatus comprises aneutron source and a source material that emits positrons in response tobombardment of the source material with neutrons. The source material isalternately positionable adjacent the neutron source and a specimen.When the source material is positioned adjacent the neutron source, thesource material is exposed to neutrons produced by the neutron source,which generates positron-producing nuclei within the source material.When the source material is positioned adjacent the specimen, thespecimen is exposed to at least some of the positrons emitted by thesource material. A detector system positioned adjacent the specimendetects annihilation gamma rays emitted by the specimen.

A non-destructive testing method according to one embodiment of theinvention comprises: providing a source material; bombarding the sourcematerial with photons so that the source material emits positrons;placing the source material adjacent a specimen to be tested; anddetecting annihilation gamma rays emitted by the specimen.

Another embodiment of a non-destructive testing method comprisesproviding a source material; bombarding the source material withneutrons so that the source material emits positrons; placing the sourcematerial adjacent a specimen to be tested; and detecting annihilationgamma rays emitted by the specimen.

BRIEF DESCRIPTION OF THE DRAWING

Illustrative and presently preferred embodiment of the invention areshown in the accompanying drawing in which:

FIG. 1 is a schematic representation of non-destructive testingapparatus according to one embodiment of the present invention;

FIG. 2 is a perspective view of one embodiment of a source material; and

FIG. 3 is a schematic representation of another embodiment ofnon-destructive testing apparatus according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Non-destructive testing apparatus 10 according to one embodiment of thepresent invention is illustrated in FIG. 1 and may comprise a photonsource 12 for producing photons 14. A source material 16 is positionedadjacent the photon source 12 so that the source material 16 isbombarded by photons 14 from the photon source 12. As will be describedin greater detail below, the source material 16 comprises a materialthat emits positrons e+in response to bombardment of the source material16 with photons, such as photons 14 from the photon source 12. Afterbeing bombarded with photons 14 from the photon source 12, a processreferred to herein in the alternative as “photon activation,” the sourcematerial 16 may be removed from a position adjacent the photon source 12and moved to a position adjacent a specimen 18. As will be described ingreater detail below, the positrons emitted by the source material 16may have relatively high energies, e.g., in the range of about 0.5 MeVto about 5 MeV. The higher energies associated with the source material16 is due in part to the higher energies of the positrons producedwithin the source material 16. The higher energies are also due in partto the fact that the source material 16 may be made quite thin incomparison with conventional isotopic sources of external positrons,such as ²²Na, thereby resulting in less energy attenuation as thepositrons escape the source material 16.

In one embodiment, the source material 16 is conformable (i.e.,flexible) so that at least a portion of the source material 16 may beconformed to at least a portion of the specimen 18, as best seen inFIG. 1. The specimen 18 is then exposed to positrons e⁺ emitted by thesource material 16. The ability to place the source material 16 in closeproximity to the specimen 18 maximizes the ability to expose thespecimen 18 to the high-energy positrons produced by the source material16. Stated another way, the specimen 18 will be exposed to morepositrons and to positrons having higher energies than would be the caseif the source material 16 could not be placed in such close proximity tothe specimen 18.

At least some of the positrons e⁺ from the source material 16 penetratethe specimen 18 and annihilate with electrons e⁻ in the specimen 18. Theannihilation of positrons e⁺ and electrons e⁻ results in the productionor formation of annihilation gamma rays γ_(a) in a process generallyknown as positron annihilation. The measured annihilation energy plus orminus the momenta of the electrons and positrons provides a measure ofthe defect density or microcracking that may be present in the specimen18. A detector 20 positioned adjacent the specimen 18 detectsannihilation gamma rays γ_(a) emitted by the specimen 18. A processingsystem 22 operatively associated with the detector 20 processes outputdata 24 from the detector and produces output data 26. Thereafter, theoutput data 26 may be presented on a user interface 28.

A non-destructive testing method according to the present invention maycomprise providing a source material 16 that will produce positrons e⁺in response to photon bombardment. The source material 16 is thenbombarded with photons (e.g., photons 14 from the photon source 12) sothat the source material 16 emits positrons e⁺. After being bombardedwith photons 14, the source material 16 is then placed adjacent thespecimen 18 to be tested. The specimen 18 is exposed at least some ofthe positrons e⁺ emitted by the source material 16. Annihilation gammarays γ_(a) resulting from the annihilation within the specimen 18 ofpositrons e⁺ with electrons e⁻ are then detected by the detector 20 andprocessed by the processing system 22. As mentioned above, the measuredannihilation energy plus or minus the momenta of the electrons andpositrons provides a measure of the defect density or microcracking thatmay be present in the specimen 18.

The output data 26 from the processing system 22 may comprise any of awide range of data derived from the detection of the annihilation gammarays γ_(a). For example, the momentum of the positron e⁺ is related tothe environment in which it resides. However, the positron thermalizesto a very low momentum state and interacts with electrons having momentadetermined by the defect level and microstructure of the specimen 18being tested. Electron momentum is relatively low in defects (e.g.,microcracks in composite materials and polymers) or in large latticestructures, whereas electron momentum is higher in defect-free or tightlattice structures. The momentum of the positron/electron (e⁺/e⁻)interaction may be measured from the degree of broadening of the 511 keVgamma-ray energy peak caused by a number annihilation events.Alternatively, the momentum of the positron e⁺ may be derived from thedeviation from 180° of the annihilation gamma rays. Additionalinformation about the annihilation event may be detected and used toderive additional or supplemental information regarding the specimenbeing tested, such as the presence within the specimen of contaminantsor pores. Accordingly, the detection of positrons and the products ofannihilation events provide much information relating to defects andother characteristics of the specimen being tested.

One advantage of the non-destructive testing apparatus according to thepresent invention is that it provides an external source ofcomparatively high energy positrons (e.g., positrons having energies ina range of about 0.5 MeV to about 5 MeV). Such high-energy positrons canpenetrate more deeply (e.g., up to several centimeters in somematerials, such as low-density polymers) into a specimen being tested.Perhaps even more advantageously, the source material 16 may be“activated” to produce positrons over substantially the entirety of thesource material 16. In other words, the source material 16 may become a“distributed positron source” or simply, “a distributed source” ofpositrons, thereby allowing a larger area of the specimen 18 to beexposed to positrons than would otherwise be the case with conventionalexternal positron point sources. Still yet other advantages of thepresent invention may be realized if the source material 16 is“conformable,” i.e., may be readily conformed or “molded” to thespecimen being tested. So conforming or molding the source material 16to the specimen 18 improves the ability to inject positrons more deeplyinto the specimen 18 and allows for a more even distribution of thosepositrons that do enter the specimen 18. For example, it has beendiscovered that a conformable source material producing higher energypositrons is more effective at bombarding the specimen with positrons,even where the specimen 18 contains surface contaminants, such asgrease, that often inhibit the bombardment of the specimens withpositrons produced by conventional external positron sources.

In addition, the source material 16 often may be readily placed at anyconvenient location on the specimen 18. For example, in addition tobeing placed adjacent a front surface or portion of a specimen 18, thesource material 16 could also be placed adjacent some other surface orportion of the specimen 18, such as a back surface or a side surface. Inaddition, measurements of the annihilation gamma rays may be made fromany of a wide range of positions around the specimen 18, furtherincreasing utility. Moreover, such measurements of the annihilationgamma rays may even be made where an intervening material (e.g., up toseveral centimeters of steel) might be located between the specimen 18and the detector system 20. The ability to place the source material 16adjacent any convenient portion of the specimen 18, as well as theability to detect annihilation gamma rays, even where an interveningmaterial exists, allows the present invention to be readily used oncomplex, fabricated structures, such as aircraft structures, without theneed to disassemble the structure or remove material that might belocated between the specimen 18 to be examined and the detector system20.

Having briefly described one embodiment of a method and apparatus fornon-destructive testing according to the present invention, as well assome of their more significant features and advantages, the variousembodiments of the method and apparatus for non-destructive testing willnow be described in detail.

Referring back now to FIG. 1, a first embodiment 10 of non-destructivetesting apparatus according to the present invention may comprise aphoton source 12 for producing photons 14 and directing the photons 14toward the source material 16. It is generally preferred, but notrequired, that the photon source 12 be capable of producing photons 14having user-selectable energies. The ability to vary the energies of thephotons 14 will allow a user to “activate” (i.e., produce nuclei thatemit positrons) source materials 16 comprising any of a wide range ofelements by selecting the appropriate photon energies. The positrons areproduced in such materials via a gamma-ray-capture or “gamma-n”reaction. For example, if the source material 16 comprises copper-63(⁶³Cu), photons having energies of at least about 8 MeV will activatethe copper-63 contained in the source material 16 by producing orforming within the source material 16 an isotope of copper (e.g., ⁶⁴Cu)via the gamma ray capture or “gamma-n” reaction. ⁶⁴Cu is a “positronemitter.” That is, ⁶⁴Cu emits positrons having energies of about 0.66MeV during decay. ⁶⁴Cu has a half-life of about 12.7 hours. Similarly,photons having energies of at least about 11 MeV will convert ⁶³Cu to⁶²Cu which emits higher energy positrons (i.e., positrons havingenergies of about 2.93 MeV) during decay. ⁶²Cu has a half-life of about9.7 minutes.

Alternatively, if the ability to selectively activate certain elementscontained in the source material 16 is not required or desired in aparticular application, the photon source 12 need not be provided withcapability to adjust the photon energy, but need only provide photonshaving energies sufficient to “activate” the source material 16 so thatit emits positrons.

In one preferred embodiment having the ability to select the energies ofthe photons 14, the photon source 12 may comprise an electronaccelerator 30 for producing a stream of accelerated electrons. In orderto produce the photons 14 used to bombard the source material 16, theaccelerated electrons are directed toward a target 32 which emits thephotons 14 in response to bombardment by the accelerated electron streamproduced by the electron accelerator 30. There is a correlation betweenthe energies of the electrons comprising the electron stream produced bythe accelerator 30 and the photons 14 produced by the target 32 inresponse to the electron bombardment. Consequently, photons 14 havingspecified maximum energies can be readily produced by selecting oradjusting the energies of the electrons contained in the electron streamproduced by the accelerator 30.

In the embodiment shown and described herein, the photons 14 produced bythe photon source 12 may be selected to have energies in the range ofabout 9 million electron volts (MeV) to about 21 MeV. Photons 14 havingenergies in this range are often referred to as high energy x-rays.

In accordance with the foregoing considerations, then, the electronaccelerator 30 may comprise a linear accelerator of the type that arenow known in the art or that may be developed in the future that are orwould be suitable for the production of electrons having any of a widerange of energies. By way of example, in one preferred embodiment, theelectron accelerator comprises a model 6000 linear accelerator availablefrom Varian Corp. of Palo Alto, Calif. Alternatively, equivalent devicesfrom the same or other manufacturers may also be used. The target 32which emits the photons 14 may comprise tungsten, although othermaterials may also be used. Of course, the photon source 12 and/or thevarious components comprising the photon source 12 (e.g., the electronaccelerator 30 and target 32) may be provided with suitable shieldingmaterials (not shown), to prevent the unwanted escape of radiation fromthe photon source 12.

The source material 16 is illustrated in FIGS. 1 and 2 and comprises asubstance that emits positrons in response to bombardment by photonshaving the appropriate energies. The photon energy required to cause thesource material 16 to emit positrons depends on the composition of thesource material 16. For example, the source material 16 must contain atleast one element that, when bombarded by photons, becomes an unstableisotope (e.g., via a “gamma-n” reaction) that decays in part by emittingpositrons. Such an unstable isotope is referred to herein as a “positronemitter.” A list of representative positron emitters, productionreactions, the threshold photon (i.e., gamma ray) energies required toform or “activate” the positron emitters, as well as their half-lives,are presented herein as Table I. Table I may be used to readily identifyrepresentative isotopes that may be converted into positron emitters byphoton bombardment, as well as to estimate the photon energies requiredto form the positron emitters. Stated simply, then, any material thatcontains at least one of isotopes presented in Table I or similarisotopes that produce positrons through photon or neutron interactionsmay be considered for use as the source material 16, although otherconsiderations described herein and/or known to persons having ordinaryskill in the art after becoming familiar with the teachings of thepresent invention may favor or disfavor its use in any particularapplication. TABLE I Threshold Element Reaction Half-Life Units EnergyMeV Chromium ⁵⁰Cr→⁴⁹Cr 42.3 Minutes 20.5 Iron ⁵⁴Fe→⁵³Fe 8.51 Minutes 14Nickel ⁵⁸Ni→⁵⁷Ni 35.6 Hours 12 Copper ⁶⁵Cu→⁶⁴Cu 12.7 Hours 8 Copper⁶³Cu→⁶²Cu 9.74 Minutes 11 Zinc ⁶⁴ Zn→⁶³Zn 38.5 Minutes 20.45 Zirconium⁹⁰Zr→⁸⁹Zr 4.18 Minutes 12.3 Molybdenum ⁹²Mo→⁹¹Mo 1.08, Minutes 12.5 15.5Tin ¹¹²Sn→¹¹¹Sn 35 Minutes 12.5 Antimony ¹²¹Sb→¹²⁰Sb 15.9 Minutes 10Titanium ⁴⁶Ti→⁴⁵Ti 3.1 Hours 13 Carbon ¹²C→¹¹C 20.3 Minutes 19 Nitrogen¹⁴N→¹³N 9.97 Minutes 10.5 Oxygen ¹⁵O→¹⁴O 122.2 Seconds ND Fluorine¹⁹F→¹⁸F 1.83 Hours 20 Phosphorus ³¹P→³⁰P 2.5 Minutes 10.9 Chlorine³⁵Ci→³⁴Ci 32.2 Minutes ND Potassium ³⁹K→³⁸K 7.6 Minutes 12.5 Gallium⁶⁹Ga→⁶⁸Ga 1.13 Hours ND Selenium ⁷⁴Se→⁷³Se 40 Minutes 12 Bromine⁷⁹Br→⁷⁸Br 6.45 Minutes ND Ruthenium ⁹⁶Ru→⁹⁵Ru 1.64 Hours ND Palladium¹⁰²Pd→¹⁰¹Pd 8.4 Hours ND Silver ¹⁰⁷Ag→¹⁰⁶Ag 24 Minutes 9.0 Cadmium¹⁰⁶Cd→¹⁰⁵Cd 55.5 Minutes ND Indium ¹¹³In→¹¹²In 14.4 Minutes ND Xenon¹²⁴Xe→¹²³Xe 2 Hours ND Cerium ¹³⁶Ce→¹³⁵Ce 17.7 Minutes ND Praseodymium¹⁴¹Pr→¹⁴⁰Pr 40 Minutes 7 Neodymium ¹⁴²Nd→¹⁴¹Nd 1.04 Minutes 9.5 Samarium¹⁴⁴Sm→¹⁴³Sm 8.83 Minutes 12.5 Europium ¹⁵¹Eu→¹⁵⁰Eu 12.8 Hours ND Erbium¹⁶⁴Er→¹⁶³Er 1.25 Hours ND

It is also generally preferred, but not required, that the sourcematerial 16 comprise a flexible material so that it may be conformed orshaped to the particular specimen 18 being tested. As already mentioned,the ability to conform or shape at least a portion the source material16 to at least a portion of the specimen 18 may provide certainadvantages. Generally speaking, source materials 16 comprising thinmetal sheets or “foils” containing at least one isotope that can be“photon activated” will result in highly-suitable source materials 16.The thickness 34 (FIG. 2) of the sheet or foil is not particularlycritical, and may comprise any of a range of thicknesses. However, ifhigh-conformability is desired, then the thickness 34 of the sourcematerial 16 should be selected to provide the source material 16 withthe desired degree of flexibility or conformability. For example, in oneembodiment, the source material 16 comprises a copper sheet or foilhaving a thickness 34 in the range of about 0.25 mm to about 1 mm (0.025mm to 0.1 mm preferred).

The sheet-like or foil source material 16 need not comprise anyparticular shape or configuration. Indeed, the source material 16 maycomprise any of a wide range of shapes or configurations (e.g.,rectangular, square, elliptical, tubular, or irregular) that may berequired or desired for the particular application, as would be obviousto persons having ordinary skill in the art after having become familiarwith the teachings of the present invention. Consequently, the presentinvention should not be regarded as limited to a source material 16having any particular shape or configuration. However, by way ofexample, in one preferred embodiment, the source material 16 maycomprise a generally square configuration, as illustrated in FIG. 2,having a length 36 that is substantially equal to a width 38. In theembodiment illustrated in FIG. 2, both the length 36 and width 38 areabout 10 cm. That is, the source material 16 comprises a sheet or“coupon” of square foil having a total surface area of about 200 cm²(i.e., about 100 cm² for each side or face 40, 42 of the source material16). Alternatively, source materials 16 having other surface areas maybe used. By way of example, source materials 16 having single-sidesurface areas ranging from about 0.1 cm² to about 100 cm² have been usedwith good results.

In the embodiment shown and described in FIG. 1, the copper foil sourcematerial 16 comprises primarily ⁶³Cu. With reference to Table I, ⁶³Cumay be “activated” or transformed into a positron emitter by bombardingthe ⁶³Cu with photons having energies of at least about 8 MeV.Bombarding the copper foil with photons having at least this energy willresult in the production of ⁶⁴Cu which is a positron emitter. That is,⁶⁴Cu emits positrons having energies of about 0.66 MeV in the decayprocess. ⁶⁴Cu has a half-life of about 12.7 hours. Alternatively, higherenergy photons, e.g., photons having energies of at least about 11 MeV,will result in the formation of ⁶²Cu, which is also a positron emitter.⁶²Cu emits positrons having energies of about 2.93 MeV. ⁶²Cu has ahalf-life of about 9.7 minutes.

It is generally preferable to “activate” as large a portion of thesource material 16 as possible in order to provide an activated sourcematerial 16 that emits positrons e⁺ over as wide an area as possible,depending on the application. Of course, the nature of the particularapplication may allow a smaller area of the source material 16 to beactivated. For example, if the source material 16 comprises a surfacearea (i.e., size) that is larger than the size or surface area of thespecimen 18 to be examined, then it may be desirable to only activate anarea of the source material 16 that approximates the area of thespecimen 18 to be examined. The activated area of the source material 16should then be aligned with (e.g., positioned over) the correspondingarea on the specimen 18 that is to be tested. In any event, thearrangement of the photon source 12 and the source material 16 should besuch that substantially the entirety, or desired portion, as the casemay be, of the source material 16 is exposed to photons 14 from thephoton source 12.

The exposure of the source material 16 to photons 14 from the photonsource 12 may be accomplished in any of a variety of ways. For example,if the photon source 12 produces a wide photon “beam” it may be that thearea of the wide photon beam is sufficient to activate substantially theentirety (or desired portion) of the source material 16. Alternatively,the source material 16 may be moved with respect to the beam of photons14 produced by the photon source 12 so that substantially the entirety(or desired portion) of the source material 16 is activated by the beamof photons 14. The relative movement may be regular (e.g., such as in a“raster” scan) or random. In any event, suitable movement apparatus 44(FIG. 1) may be used to move the source material 16 with respect to thephoton source 12. However, because apparatus for moving the sourcematerial 16 with respect to the photon source 12 could be easilyprovided by persons having ordinary skill in the art after having becomefamiliar with the teachings of the present invention, the particularapparatus, such as movement apparatus 44, that may be used to exposesubstantially the entirety (or desired portion) of the source material16 to the photons 14 from photon source 12 will not be described infurther detail herein.

The detector system 20 may be positioned adjacent the specimen 18 sothat the detector system 20 receives annihilation gamma rays γ_(a)resulting from positron/electron annihilation events occurring withinthe specimen 18. Depending on the geometry of the particularinstallation, a shield (not shown) may be positioned between the photonsource 12 and the detector system 20 to prevent gamma radiation from thephoton source 12 from being detected by the detector system 20. However,in most cases, the photon source 12 will be far removed from thespecimen 18 and/or inactive during the time the detector system 20 isdetecting annihilation gamma rays γ_(a). The detector system 20 maycomprise any of a wide range of gamma ray detectors that are now knownin the art or that may be developed in the future at are or would besuitable for detecting annihilation gamma rays γ_(a) produced by theannihilation of positrons and electrons within the specimen 18.Accordingly, the present invention should not be regarded as limited toa detector system 20 comprising any particular type of gamma raydetector. However, by way of example, in one preferred embodiment, thedetector 20 may comprise a germanium detector of the type that iswell-known in the art and readily commercially available. Alternatively,the detector 20 could comprise a cadmium-zinc-tellurium or BaF₂ detectorof the type that is also known in the art and readily commerciallyavailable.

It should also be noted that the detector system 20 should not beregarded as limited to a single detector. Indeed, depending on the typeof processing algorithms utilized by the processing system 22 it may bedesirable, or even required, that the detector system 20 comprise morethan a single gamma ray detector. For example, while a Dopplerbroadening algorithm (described in greater detail below) that may beutilized by the processing system 22 will generally not require the useof two or more detectors, other algorithms, such as position locatingalgorithms, that may be implemented by the processing system 22 mayrequire the use of at least two, and possibly several, gamma raydetectors 20 in order to determine the positions within the specimen 18of the positron/electron annihilation events. However, sinceannihilation position determining techniques are well-known in the art,as are the requirements for the particular types and positions ofdetectors associated with such techniques, and since such multipledetectors could be easily provided by persons having ordinary skill inthe art after having become familiar with the teachings of the presentinvention, the particular configurations of such multiple detectorsystems as they could be utilized in the present invention will not bedescribed in greater detail herein.

The gamma ray detector (or detectors) comprising the detector system 20may also be provided with a collimator 46 to collimate the annihilationgamma rays γ_(a). The collimator 46 may comprise any of a wide range ofcollimators, such as, for example, a variable-slit type collimator, thatare now known in the art or that may be developed in the future.However, because collimators are well-known in the art and could bereadily provided by persons having ordinary skill in the art afterhaving become familiar with the teachings of the present invention, theparticular collimator 46 that may be used with the detector 20 will notbe described in further detail herein.

The processing system 22 is operatively associated with the detectorsystem 20 and receives output data 24 produced by the detector system20. As mentioned above, the output data 24 produced by the detectorsystem 20 may be processed in accordance with any of a wide variety ofalgorithms that are now known in the art or that may be developed in thefuture to allow the processing system 22 to produce output data 26indicative of at least one material characteristic of the specimen 18being tested. Algorithms that may be utilized by the data processingsystem include a Doppler broadening algorithm, such as the typedisclosed in U.S. Pat. No. 6,178,218 issued to Akers, which isincorporated herein by reference for all that is disclosed. Otheralgorithms that may be utilized are disclosed in U.S. patent applicationSer. No. 09/932,531, filed Aug. 17, 2001, and entitled “Apparatus forPhoton Activation Positron Analysis,” to Akers which is incorporatedherein by reference for all that it discloses. In addition, algorithmssuitable for use in positron annihilation are also disclosed in“Positron-Annihilation Spectroscopy”, Encyclopedia of Applied Physics,Vol. 14, pp. 607-632 (1996), which is also incorporated herein byreference for all that it discloses. Because algorithms suitable forprocessing data related to the annihilation gamma rays detected by thedetector system 20 are known in the at and could be easily provided bypersons having ordinary skill in the art after having become familiarwith the teachings of the present invention, the particular algorithm oralgorithms that may be utilized by the processing system 22 will not bedescribed in further detail herein.

The processing system 22 may comprise any of a wide range of systems orcombinations of systems known in the art that would be suitable forcollecting the output data 24 from the detector system 20 and forprocessing the output data 24 in accordance with the one or morealgorithms described above. Therefore, the present invention should notbe regarded as limited to processing systems 22 comprising anyparticular type or configuration. By way of example, in one embodiment,the processing system 22 comprises a general purpose programmablecomputer system, such as the ubiquitous personal computer. However,because such processing systems are well known in the art and could beeasily provided by persons having ordinary skill in the art afterbecoming familiar with the teachings of the present invention and afterconsidering the particular type or types of algorithms to be used toprocess the data 24 from the detector system 20 to produce the outputdata 26, the processing system 22 utilized in one preferred embodimentof the invention will not be described in further detail herein.

The processing system 22 may be connected to a suitable user interface28, such as a flat-panel display (not shown), for displaying the outputdata 26 produced by the processing system 22. Alternatively, theprocessing system 22 could also provide the output data 26 incomputer-readable form for subsequent display and/or manipulation by adevice external to the processing system 22, such as a personal computer(not shown). However, because such user-interfaces are well-known in theart and could be easily provided by persons having ordinary skill in theart after having become familiar with the teachings provided herein andafter considering the particular application, the present inventionshould not be regarded as limited to any particular type ofuser-interface 28.

A non-destructive testing method according to the present invention maycomprise providing a source material 16 that will produce positrons e⁺in response to photon bombardment. In the embodiment shown and describedin FIG. 1, the source material 16 comprises a thin sheet or foil“coupon” comprising ⁶³Cu having a generally square configuration,substantially as illustrated in FIG. 2. More specifically, the coppersource material 16 comprises a foil sheet having a thickness 34 of about0.025 mm, a length 36 of about 10 mm and a width 38 of about 10 mm. Thecopper foil comprising the source material 16 is then positionedadjacent the photon source 12, which bombards the source material 16with photons 14. As mentioned above, the source material 16 may bemounted on a suitable movement apparatus 44 which moves the sourcematerial 16 relative to the photons 14 from photon source 12 so that thedesired portion of the source material 16 may be exposed to the photons14.

Once the source material 16 is positioned adjacent the photon source 12,the photon source 12 may then be operated so that the photons 14produced thereby have energies in the range of about 9 MeV to about 21MeV. Photons 14 of such energies are sufficient to activate the sourcematerial 16, causing it to produce positrons e⁺ in the manner alreadydescribed.

The time required to activate the source material 16 will vary dependingon the size of the source material 16 that is to be used in a particularapplication, as well as on other factors, as would become apparent topersons having ordinary skill in the art after having become familiarwith the teachings provided herein and after considering the particulardevices and materials being used. Consequently, the present inventionshould not be regarded as limited to exposing the source material 16 tophotons 14 for any particular time. However, by way of example, in onepreferred embodiment, the source material 16 is exposed to the photons14 from the photon source 12 for a time in the range of about 0.5 toabout 3 minutes, which is sufficient to activate substantially theentirety of the source material 16 in the embodiment shown and describedherein.

After being activated, the source material 16 may then be removed fromits position adjacent the photon source 12 and positioned adjacent thespecimen 18 to be tested. Generally speaking, it will be advantageous toplace the source material 16 in direct contact with the specimen 18. Ifthe specimen 18 comprises an irregularly shaped body and if the sourcematerial 16 comprises a conformable material, then it also will beadvantageous to conform or mold the source material 16 to the specimen18, as best seen in FIG. 1. So conforming or molding the source material16 to the specimen 18 is readily accomplished in the embodimentillustrated in FIG. 1, wherein the source material 16 comprises copperfoil.

When so positioned adjacent the specimen 18, positrons e⁺ emitted by theactivated source material 16 bombard the specimen 18. In manyembodiments, the positrons e⁺ will have energies (e.g., in the range ofabout 2 MeV to about 5 MeV) greater than those normally associated withconventional external positron sources (e.g., about 0.5 MeV), thus willgenerally penetrate the specimen 18 to a greater depth. The sourcematerial 16 may be left in place adjacent the specimen 18 for a timesufficient to allow the detector system 20 to collect an amount ofannihilation gamma rays γ_(a) sufficient to allow the processing system22 to produce output data 26. In this regard, it should be noted thatthe amount of annihilation gamma rays γ_(a) needed to be detected by thedetector system 20 will vary depending on a wide variety of factors,such as, for example, the sensitivity of the detector system 20, theparticular algorithms utilized by the processing system 22, as well ason other factors that would be appreciated by persons having ordinaryskill in the art. Therefore, the present invention should not beregarded as limited to any particular times. However, by way of example,in one embodiment, the source material 16 is positioned adjacent thespecimen for a time in the range of about 3 minutes to about 10 minutes.

The source material 16 may be left in place during the data collection(i.e., detection) process. Alternatively, the source material 16 may beremoved before conducting the data collection process. In one embodimentwherein the source material 16 is left in place, the source material 16is placed on the side of the specimen 18 that is opposite the detectorassembly 20. Alternatively, the source material 16 may be placed on thesame side as the detector assembly 20. In this case, it may be desirableto provide the detector assembly 20 with suitable shielding (e.g., acover foil) to prevent the positrons emitted by the source material 16from interfering with the detector assembly 20.

Another embodiment 110 of non-destructive testing apparatus according tothe present invention is illustrated in FIG. 3. The second embodiment110 differs from the first embodiment 10 in that the second embodiment110 involves the activation of a source material 116 by neutrons ninstead of photons 14. The neutrons n may be produced by a suitableneutron source 112. It is generally preferred, but not required, thatthe neutron source 112 be capable of producing neutrons n havingenergies of about 14 MeV. Alternatively, the neutron source 112 couldcomprise a neutron source capable of producing neutrons havinguser-selectable energies. The ability to vary the energies of theneutrons n will allow a user to activate (i.e., cause to emit positrons)source materials 116 comprising various elements by selecting theappropriate neutron energies. The positrons are produced in suchmaterials via either an (n,2n) reaction or an (n,γ) reaction. Materialscapable of producing positrons e⁺ in response to neutron bombardmentinclude many of those produced using photons. It is generally preferredthat the neutron source 112 produce neutrons n having energies of up toabout 14 MeV for (n,2n) reactions. Alternatively, lower energy neutronscan be used for other reactions.

It should be noted that neutrons n from the neutron source 112 mayactivate other materials within the source material 116, causing them tobecome positron emitters through the process of pair production.However, this only happens of the energies of the prompt gamma rays(emitted during pair production) exceed about 1.1 MeV. Prompt gamma rayshaving energies less than about 1.1 MeV typically result in theproduction of delayed gamma rays and, occasionally, electrons, but notpositrons.

The neutron source 112 may comprise any of a wide range of devicessuitable for producing neutrons having sufficient energies to activatethe source material 116. By way of example, in one preferred embodiment,the neutron source 112 comprises a model no. A-320 neutron sourceavailable from the MF Physics Corporation of a subsidiary of the ThermoElectron Corporation. Alternatively, the neutron source 112 couldcomprise an isotopic source, such as ²⁵²Cf.

The source material 116 comprises a substance that emits positrons inresponse to bombardment by neutrons having the appropriate energies. Theneutron energy required to cause the source material 116 to emitpositrons depends on the composition of the source material 116. Thatis, the source material 116 must contain at least one element that, whenbombarded by neutrons, becomes an unstable isotope that decays in partby emitting positrons. The neutron energies required to activate certainelements are known, thus will not be provided herein. However, by way ofexample, if the source material 116 comprises ⁶³Cu, bombardment of thesource material 116 by neutrons n having energies of about 14 MeV willresult in the production of ⁶⁴Cu via the (n-γ) reaction process. ⁶⁴Cu isa “positron emitter” (i.e., decays by emitting positrons) having ahalf-life of about 12.7 hours. Also, the ⁶³Cu (n,2n) reaction produces⁶²Cu, a positron emitter having a half-life of 9.7 minutes. Statedsimply, then, any material that contains at least one element that iscapable of becoming a positron emitter in response to neutronbombardment may be selected as the source material 116. For example,suitable source materials 116 may comprise copper and various alloysthereof, as well as many aluminum and steel alloys.

As was the case for the source material 16, it is generally preferred,but not required, that the source material 116 comprise a flexiblematerial so that it may be conformed or shaped to the particularspecimen 118 being tested. As discussed above, the ability to conform orshape at least a portion the source material 1 16 to at least a portionof the specimen 118 may provide certain advantages. Generally speaking,source materials 116 comprising thin metal sheets or “foils” containingat least one isotope that can be “neutron activated,” such as copper, aswell as certain aluminum and steel alloys, will result inhighly-suitable source materials 116. With reference back now to FIG. 2,the thickness 34 of the sheet or foil is not particularly critical, anymay comprise any of a range of thicknesses, although the sensitivity isgreater with thinner source materials. However, if high-conformabilityis desired, then the thickness 34 of the source material 116 should beselected to provide the source material 116 with the desired degree offlexibility or conformability. For example, in one embodiment, thesource material 116 comprises a copper sheet or foil having a thickness34 in the range of about 0.025 mm to about 1 mm (0.025 mm to 0.1 mmpreferred).

The sheet-like or foil source material 116 need not comprise anyparticular shape or configuration. Indeed, the source material 116 maycomprise any of a wide range of shapes or configurations (e.g.,rectangular, square, elliptical, or irregular) that may be required ordesired for the particular application, as would be obvious to personshaving ordinary skill in the art after having become familiar with theteachings of the present invention. Consequently, the present inventionshould not be regarded as limited to a source material 116 having anyparticular shape or configuration. However, by way of example, in onepreferred embodiment, the source material 116 may comprise a generallysquare configuration, as illustrated in FIG. 2, having a length 36substantially equal to a width 38, both of which are about 10 cm toyield a total surface area of about 200 cm² (e.g., 100 cm² for each sideor face 40, 42 of the source material 116). However, and as mentionedabove, source materials having single side surface areas in the range ofabout 0.1 cm² to about 100 cm² may be used as well.

In the embodiment shown and described in FIG. 3, the copper foil sourcematerial 116 comprises primarily ⁶³Cu. ⁶³Cu may be “activated” or causedto emit positrons by bombarding it with neutrons having energies of atleast about 14 MeV. Bombarding the copper foil with neutrons having atleast this energy will result in the formation or production of ⁶⁴Cu(e.g., via the (n,γ) reaction) and ⁶²Cu (e.g., via the (n,2n) reaction),both of which emit positrons in the decay process.

As was the case for the first embodiment, it is generally preferable to“activate” as large a portion of the source material 116 as possible inorder to provide an activated source material 116 that emits positronse⁺ over as large an area as possible, or in a size to produce thedesired resolution for a particular measurement. Of course, theparticular application may allow a smaller area of the source material116 to be activated. For example, if the source material 116 comprises asurface area (i.e., size) that is larger than the size or surface areaof the specimen 118 to be examined, then it may be desirable to onlyactivate an area of the source material 116 that approximates the areaof the specimen 118 to be examined. The activated area of the sourcematerial 116 should then be aligned with (e.g., positioned over) thecorresponding area on the specimen 118 that is to be tested. In anyevent, the arrangement of the neutron source 112 and the source material116 should be such that substantially the entirety, or desired portion,as the case may be, of the source material 116 is exposed to neutrons nfrom the neutron source 112.

The exposure of the source material 116 to neutrons n from the neutronsource 112 may be accomplished in any of a variety of ways. For example,if the neutron source 112 produces a wide neutron “beam” it may be thatthe area of the wide neutron beam is sufficient to activatesubstantially the entirety (or desired portion) of the source material116. Alternatively, the source material 116 may be moved with respect tothe neutron beam produced by the neutron source 112 so thatsubstantially the entirety (or desired portion) of the source material116 is activated by the beam of neutrons. The relative movement may beregular (e.g., such as in a “raster” scan) or random. In any event,suitable movement apparatus 144 may be used to move the source material116 with respect to the neutron source 112.

The non-destructive testing apparatus 110 may also be provided with adetector system 120, a processing system 122, as well as a userinterface 128. These systems may be in every respect identical to thecorresponding systems already described for the first embodiment 10 andillustrated in FIG. 1. Consequently, the detector system 120, processingsystem 122, and user interface 128 that may be utilized in oneembodiment of the apparatus 110 will not be described in further detailherein.

Another non-destructive testing method according to the presentinvention may comprise providing a source material 116 that will producepositrons e⁺ in response to neutron bombardment. In the embodiment shownand described in FIG. 3, the source material 116 comprises a thin sheetor foil comprising ⁶³Cu having a generally square configuration,substantially as illustrated in FIG. 2. More specifically, the coppersource material 116 comprises a foil sheet having a thickness 34 ofabout 0.1 mm, a length 36 of about 10 mm and a width 38 of about 10 mm.The copper foil comprising the source material 116 is then positionedadjacent the neutron source 112, which bombards the source material 116with neutrons n. As mentioned above, the source material 116 may bemounted on a suitable movement apparatus 144 which moves the sourcematerial 116 relative to the neutrons n from neutron source 112 so thatthe desired portion of the source material 116 is exposed to theneutrons n.

Once the source material 1 16 is positioned adjacent the neutron source112, the neutron source 112 may then be operated so that the neutrons nproduced thereby have energies of about 14 MeV. Neutrons n of suchenergies are sufficient to activate the copper atoms in the sourcematerial 116, causing it to produce positrons e⁺ in the manner alreadydescribed.

The time required to activate the source material 116 will varydepending on the size of the source material 116 that is to be used in aparticular application, as well as on other factors, as would becomeapparent to persons having ordinary skill in the art after having becomefamiliar with the teachings provided herein and after considering theparticular devices and materials being used. Consequently, the presentinvention should not be regarded as limited to exposing the sourcematerial 116 to neutrons n for any particular time. However, by way ofexample, in one preferred embodiment, the source material 116 is exposedto the neutrons n from the neutron source 112 for a time in the range ofabout 1 to about 20 minutes, which was sufficient to activatesubstantially the entirety of the source material 116.

After being activated, the source material 116 may then be removed fromits position adjacent the neutron source 112 and positioned adjacent thespecimen 118 to be tested. Generally speaking, it will be advantageousto place the source material 116 in direct contact with the specimen118. If the specimen 118 comprises an irregularly shaped body and if thesource material 116 comprises a conformable material, then it also willbe advantageous to conform or mold the source material 16 to thespecimen 118, as illustrated in FIG. 3. So conforming or molding thesource material 116 to the specimen 118 is readily accomplished in theembodiment illustrated in FIG. 3, wherein the source material 116comprises copper foil.

When so positioned adjacent the specimen 118, positrons e⁺ emitted bythe activated source material 116 bombard the specimen 118. In manyembodiments, the positrons e⁺ will have energies greater than thosenormally associated with conventional external positron sources, thuswill generally penetrate the specimen 118 to a greater depth. The sourcematerial 116 may be left in place adjacent the specimen 118 for a timesufficient to allow the detector system 120 to collect an amount ofannihilation gamma rays γ_(a) sufficient to allow the processing system122 to produce output data 126. Because the amount of annihilation gammarays γ_(a) needed to be detected by the detector system 120 will varydepending on a wide variety of factors, such as, for example, thesensitivity of the detector system 120, the particular algorithmsutilized by the processing system 122, as well as on other factors thatwould be appreciated by persons having ordinary skill in the art, thepresent invention should not be regarded as limited to any particulartimes. However, by way of example, in one embodiment, the sourcematerial 116 is positioned adjacent the specimen for a time in the rangeof about 3 minutes to about 60 minutes.

Having herein set forth preferred embodiments of the present invention,it is anticipated that suitable modifications can be made thereto whichwill nonetheless remain within the scope of the invention. The inventionshall therefore only be construed in accordance with the followingclaims:

1. Non-destructive testing apparatus, comprising: a photon source; asource material, said source material emitting positrons in response tobombardment of said source material with photons from said photonsource, said source material being positionable adjacent a specimen, sothat the specimen is exposed to at least some of the positrons emittedby said source material, the specimen emitting annihilation gamma raysin response to exposure to the at least some of the positrons emitted bysaid source material; and a detector system, said detector system beingpositionable adjacent the specimen, said detector system detectingannihilation gamma rays emitted by the specimen.
 2. The non-destructivetesting apparatus of claim 1, wherein said source material isconformable so that at least a portion of said source material may beconformed to at least a portion of the specimen.
 3. The non-destructingtesting apparatus of claim 1, wherein said source material comprisescopper.
 4. The non-destructive testing apparatus of claim 1, whereinsaid source material has a thickness in a range of about 0.025 mm toabout 1 mm.
 5. The non-destructive testing apparatus of claim 1, whereinsaid source material has a surface area in the range of about 0.1 cm² toabout 100 cm².
 6. The non-destructive testing apparatus of claim 1,wherein said photon source produces photons having energies in a rangeof about 9 MeV to about 21 MeV.
 7. The non-destructive testing apparatusof claim 1, wherein said source material emits positrons having energiesin a range of about 0.5 MeV to about 5 MeV.
 8. Non-destructive testingapparatus, comprising: a neutron source; a source material, said sourcematerial emitting positrons in response to bombardment of said sourcematerial with neutrons from said neutron source, said source materialbeing positionable adjacent a specimen so that the specimen is exposedto at least some of the positrons emitted by said source material, thespecimen emitting annihilation gamma rays in response to exposure to theat least some of the positrons emitted by said source material; and adetector system, said detector system being positionable adjacent thespecimen, said detector system detecting annihilation gamma rays emittedby the specimen.
 9. The non-destructive testing apparatus of claim 8,wherein said source material is conformable so that at least a portionof said source material may be conformed to at least a portion of thespecimen.
 10. The non-destructing testing apparatus of claim 8, whereinsaid source material comprises copper.
 11. The non-destructive testingapparatus of claim 8, wherein said source material has a thickness in arange of about 0.025 mm to about 1 mm.
 12. The non-destructive testingapparatus of claim 8, wherein said source material has a surface area inthe range of about 0.1 cm² to about 100 cm².
 13. The non-destructivetesting apparatus of claim 8, wherein said neutron source producesneutrons having energies of about 14 MeV.
 14. The non-destructivetesting apparatus of claim 8, wherein said source material emitspositrons having energies in a range of about 0.5 MeV to about 5 MeV.15. A non-destructive testing method, comprising: providing a sourcematerial; bombarding the source material with photons so that the sourcematerial emits positrons; placing the source material adjacent aspecimen to be tested; and detecting annihilation gamma rays emitted bythe specimen.
 16. The method of claim 15, further comprising moving thesource material away from the specimen before detecting annihilationgamma rays.
 17. The method of claim 15, wherein placing the sourcematerial adjacent a specimen to be tested comprises placing the sourcematerial in contact with the specimen.
 18. The method of claim 15,wherein placing the source material adjacent a specimen to be testedcomprises conforming at least a portion of the source material to atleast a portion of the specimen.
 19. The method of claim 15, whereinbombarding the source material with photons comprises bombarding thesource material with photons having energies in a range of about 9 MeVto about 21 MeV.
 20. The method of claim 15, wherein positrons emittedby the source material have energies in a range of about 0.5 MeV toabout 5 MeV.
 21. A non-destructive testing method, comprising: providinga source material; bombarding the source material with neutrons so thatthe source material emits positrons; placing the source materialadjacent a specimen to be tested; and detecting annihilation gamma raysemitted by the specimen.
 22. The method of claim 21, further comprisingmoving the source material away from the specimen before detectingannihilation gamma rays.
 23. The method of claim 21, wherein placing thesource material adjacent a specimen to be tested comprises placing thesource material in contact with the specimen.
 24. The method of claim21, wherein placing the source material adjacent a specimen to be testedcomprises conforming at least a portion of the source material to atleast a portion of the specimen.
 25. The method of claim 21, whereinbombarding the source material with neutrons comprises bombarding thesource material with neutrons having energies of about 14 MeV.
 26. Themethod of claim 21, wherein positrons emitted by the source materialhave energies in a range of about 0.5 MeV to about 5 MeV. 27.Non-destructive testing apparatus, comprising: photon source means forproducing photons; source material means for emitting positrons inresponse to bombardment of said source material means with photons andfor being positionable adjacent a specimen so that the specimen isexposed to at least some of the positrons emitted by said sourcematerial means; and detector means positioned adjacent the specimen fordetecting annihilation gamma rays emitted by the specimen. 28.Non-destructive testing apparatus, comprising: neutron source means forproducing neutrons; source material means for emitting positrons inresponse to bombardment of said source material means with neutrons andfor being positionable adjacent a specimen so that the specimen isexposed to at least some of the positrons emitted by said sourcematerial means; and detector means positioned adjacent the specimen fordetecting annihilation gamma rays emitted by the specimen.